Dynamic Duo

Fig. 1. This screenshot shows parallel compression applied to drum tracks in Pro Tools. Sends are used to route the drum tracks to an Aux track labeled “CmprDrms” (Compressed Drums). This track has a compressor (Kramer PIE) plug-in inserted, creating a new compressed version of the drums which is mixed in with the original drum sounds for a unique effect.

In the July issue of Electronic Musician, we
looked at basic dynamic processing. This
month, we’ll explore advanced techniques for
compression.

Watch Yer’ Bottom End Many
compressors feature a “sidechain,” which
acts sort of like a remote-control path.
The compressor “listens” to the sidechain,
compressing audio based on the sidechain
input. Usually, the signal in the sidechain is
the same signal being compressed, but what
if we could trigger compression by sending a
different signal to the sidechain?

Here’s an example: You patch a compressor
on a guitar channel and use an aux send to
route the lead vocal to that compressor’s
sidechain. Now the vocal controls compression
on the guitar track. Every time the vocal
enters, the guitar level is automatically ducked
down a bit (thus the term “ducker”). Attack
and release times are usually set fast, so that
as soon as the vocal starts, the guitar level
drops and vice-versa. This technique is used
in radio commercials to create a “donut”—an
ad spot where music automatically drops to
background levels when the voiceover enters
and becomes louder when the voiceover
ends. You can use that technique in karaoke
situations where you want the MC’s voice to
duck the music.

Sidechain filtering is useful when
compressing signals with strong content in a
particular frequency range. If you compress
the L/R mix of a dance track, you may find that
every time the kick drum hits, it sucks down
the level of the lead vocal. That’s because the
compressor is sensitive to the low-frequency
energy from the kick. The cure is to apply a
filter to the sidechain and remove some of
the low frequencies (say everything below
200Hz). Filtering the bottom end stops the
compressor from reacting every time a kick
drum hits.

A de-esser is actually a compressor with
an EQ’d sidechain. The sidechain has an
EQ boost in the upper mids and highs (say
anywhere from 3.5kHz up to 8 or 9kHz),
making the compressor very sensitive to
sibilance. Every time there’s an “s” sound, the
compressor attacks it and brings it down, but
other sounds do not trigger compression. It’s
worth repeating that the sidechain is not the
audio path. When you see a button labeled
“sidechain listen” on a compressor, it lets you
temporarily hear the signal that is triggering
compression.

Parallel Worlds Another interesting
technique is known as parallel compression,
whereby multiple versions of the same
signal are processed differently. A popular
application is to have two versions of the
drums in a mix: the ‘normal’ version and
then a super-compressed version. Adding the
squashed signal into the mix helps stabilize
the drum kit’s level and adds body without
sucking the life out of them. If you want to
get nuts, you can compress for distortion as
discussed in the July issue. With a hardware
console, you’ll need to route the drums to the
L/R master as well as to a bus or aux out. (In
live applications, I find it easier to use a bus
output—two for stereo.) In the case of the bus
out, patch the compressor on the bus insert
and set the compressor for a low threshold,
fast attack and release, and high ratio. Assign
the bus faders to the L/R master and mix to
taste. You can also use an aux send to feed
the drums to a compressor and return the
compressor back into an unused channel
where you can add EQ; if you crank the lows
at 100Hz and the highs at 10kHz on the
compressed signal, you have what is known as
the New York Compression Trick.

To do this in a DAW, create an aux send
on each channel and route it to a new aux
input channel (see Figure 1 on page 98).
Insert a compressor on the aux channel
and feed the drums to the aux sends. The
aux sends are set pre-fader so that moving
the drum channel faders does not change
the compression. Set the compressor to
squash, and you’re in business. In a DAW
or digital console, you’ll have to beware
of latency, which can at times cause phase
problems between the original tracks
and the compressed tracks. This is due to
the fact that the compressed versions
are processed an extra step, which may
result in a slight delay. If the DAW has
automatic delay compensation, turn it on.
If that doesn’t solve the problem, record
the processed signal, zoom way in on the
original and processed tracks, and manually
move the compressed track until it lines up
with the original.

Next time we’ll check out advanced gating
techniques.

Steve La Cerra is an independent audio
engineer based in NY. In addition to being
an Electronic Musician contributor, he
mixes front-of-house for Blue Oyster Cult
and teaches audio at Mercy College White
Plains campus.